Over the recent years, there has been utilized a system that executes light wavelength division multiplexing (WDM: Wavelength Division Multiplexing) of 100 or more waves at a transmission speed of 10 Gbps. A wavelength-division-multiplexing transmission is defined as a system for multiplexing and transmitting optical signals having different wavelengths through a single length of optical fiber.
Generally in a communication line, a signal route (including a transmission path, a transmission device and an in-device built-up package) is often given redundancy. There have hitherto been many cases in which a redundant system is built up by electrical processes in a time-division multiplexer (TDM: Time Division Multiplexing), etc. connected to the subordinate of a wavelength division multiplex transmitting device in a network including the wavelength division multiplexing transmission. In the recent years, however, as a multiplexing level of the light wavelength division multiplexer gets extended, there is increasingly a demand for constructing the signal redundant system batchwise at a level of the optical signals.
FIG. 4 shows one example of a system that gains the redundancy of the optical signals on a low-speed side before and after demultiplexing in the network including a wavelength division multiplexing transmission path by a light wavelength division multiplexer (a terminal device) according to the prior art.
This example takes a so-called reception terminal switching configuration, wherein the signals divided by half through a photo coupler C1′ in a transmission terminal are inputted respectively to wavelength division multiplexers/demultiplexers A in two systems, etc., and one of reception outputs in the two systems is selected by an optical switch unit 400′ on the receiving side and sent to a subordinate device. In FIG. 4, a transponder 111′ (211′) is a transponder that converts the normal optical signal inputted from the subordinate device into an optical signal having a wavelength based on ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) grid, etc. applied to the wavelength division multiplexing. The signal converted by the transponder 111′ is, after being wavelength-division-multiplexed by a multiplexing unit 112′ (212′), amplified at a level suited to the optical transmission by an optical amplifier 113′ (213′) and sent to a transmission path #1 (#2). The optical signal passing through the transmission path is, after its decreased optical level has been compensated by a reception optical amplifier 121′ (221′), wavelength-demultiplexed by a wavelength demultiplexing unit 122′ (222′). The wavelength-demultiplexed signal is sent to a transponder 123′ (223′). The transponder 123′ (223′) is a transponder including an OE (Optical-Electrical (photoelectric) converting unit) having durability against an ASE (Amplified Spontaneous Emission) noise in the output of the optical amplifier, and the input optical signal is converted into a signal having a normal optical output specified wavelength that is on the order of STM-1 (x=1 to 64), etc. and also power, and is then optically outputted.
FIG. 5 is a detailed view of the reception side including a switch 401′ that performs light receiving terminal switching based on the prior art that is omitted in FIG. 4. The transponder 123′ (223′) monitors input cut-off detection, out-of-frame detection and error performance by B1 monitoring. The switch unit 400′ conducts the input cut-off detection. For example, it is assumed that a 0-system is defined as an active system, while a 1-system is defined as a standby system, and the switch unit 400′ is to select the input from the 0-system. In an initial state, both of the 0- and 1-systems are in a state where there are none of the optical input cut-off, the out-of-frame and the occurrence of error in the switch unit 400′ as well as in the transponder 123′ (223′). If there occurs from this state a switching trigger (the optical input cut-off, the out-of-frame and an excess of an error threshold value in the transponder 123′ (223′), and the optical input cut-off in the switch unit 400′) in the active system, a control unit 407′ of the switch unit 400′ receiving this information switches over the switch selection to the standby system on condition that there is not any alarming state (the optical input cut-off, the out-of-frame and the excess of the error threshold value, and the optical input cut-off in the SW unit) in the standby system. The switch selection is switched over to the 1-system as the standby system, and a G-light output of the switch unit 400′ becomes an output in the 1-system. Then, a signal transfer down to the subordinate device is recovered after detecting the switching trigger and a delay of the switch control.
The systems exemplified in the prior art involve using optical switch devices for signal switch units. This switch device system is exemplified such as a system of mechanically switching over an optical path, a system of switching over the optical path with a photomagnetic effect (Kerr effect, etc.), and so on. These types of switches generally have no monitor function capable of checking an operation state by feedback, and therefore, if using this switch for switching of the optical signal, it is impossible to, though capable of grasping a try-to-control-state, acquire information showing which side of system is actually selected. In a transmission system having a redundant system, the grasp of the present selection system is indispensable for a trouble shifting operation on the transmission path. It is, however, impossible to grasp the final system selection due to a characteristic of the optical switch 401′ described above, and this is a serious problem in terms of administering the network. As shown in FIG. 6, supposing that a fault occurs in the optical switch 401′, in spite of the control unit 407′ trying to get the optical signal from the 0-system outputted, the optical signal from the 1-system is actually outputted, in which case an unexpected disconnection of the line might be induced if conducting the trouble shifting operation about the 1-system.
A path monitoring device disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 10-107772 is proposed as a technology related to the fault monitoring function involving a monitor of the wavelength division multiplexing light. According to this path monitoring device, monitoring of abnormality of a multiplexer or a switch is actualized by the monitor of the wavelength division multiplexing light. Further, a monitor control device disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 11-346202 is proposed as a similar technology. According to this monitor control device, the monitoring of the abnormality of the multiplexer is actualized by the monitor of the wavelength division multiplexing light.